1. Field of the Invention
The present invention relates to a sheet conveying device and an image forming apparatus.
2. Description of the Related Art
In the commercial printing business, when printing by small lots, multiple types, multiple data, etc., instead of using the conventional offset printing machine, POD (Print On Demand) by an image forming apparatus using an electrophotographic method, etc. is increasingly being applied. In order to meet such demand, for example, image forming apparatuses of an electrophotographic method are required to have precision in the front/back estimation and evenness in images, which are equal to those of an offset printing machine.
The factors causing the deviation in the front/back estimation in the image forming apparatus can be largely divided into registration errors in the vertical direction/horizontal direction, skew errors between the recording medium and the print image, and the expansion and contraction in the length of the image when transferring a toner image. Furthermore, in an image forming apparatus including a fixing device, a deviation in the front/back estimation is caused by the error in the magnification ratio of the image, according to the expansion and contraction in the recording medium when heated by the fixing device.
Such a deviation in the front/back estimation can be prevented by, for example, measuring the length of the sheet in the conveying direction before and after fixing the toner image, and correcting the image to be printed on the back side of the sheet based on the expansion/contraction ratio of the sheet.
Accordingly, there is disclosed a sheet length measuring device for detecting, with a rotary encoder, the rotation amount of a length measuring roller that is rotated by being in contact with a conveyed sheet, and calculating the length of the sheet in the conveying direction based on the detected rotation amount of the length measuring roller (see, for example, Patent Document 1).
Patent Document 1: Japanese Laid-Open Patent Publication No. 2011-6202
However, in the sheet length measuring device of Patent Document 1, in order to cause the length measuring roller to be rotated by being in contact with sheets of various thicknesses, for example, the length measuring roller is configured to be movable in the thickness direction of the sheet. In such a configuration, for example, the, rotary encoder is fixed to the apparatus frame, and is connected to the rotational shaft of the length measuring roller by a universal joint, etc., to detect the rotation amount of the rotational shaft that rotates along with the length measuring roller.
In the case of the above configuration, for example, a universal joint is required between the length measuring roller and the rotary encoder, and therefore the cost may increase. Furthermore, space is required for connecting the length measuring roller with the rotary encoder, and therefore the size of the apparatuses may increase.